Conventional aqueous micellar delivery systems have been used predominantly in the pharmaceutical industry to provide both controlled delivery of drugs and controlled release of pharmaceutical agents. A micellar solution is one that contains at least one surfactant at a concentration greater than the critical micelle concentration (“CMC”). In the case of aqueous micellar solutions, when a hydrophobic or less water soluble material such as an oil is emulsified in the micellar solution, an emulsion results. Due to the often high surfactant concentrations used in many emulsions, the resulting surfactant stabilized emulsion droplets are often very stable. The good stability against coalescence also makes emulsion droplets ideal carriers for other materials. This technology is typically used in the pharmaceutical industry for controlled delivery of pharmaceutical agents such as antibiotics, antimicrobials, antivirals, cardiovascular and renal agents. These agents are commonly incorporated into the hydrophobic component of the carrier emulsion. Frequently, such emulsions are comprised of a hydrophobic material selected from the group consisting of a long chain carboxylic acid, long chain carboxylic acid ester, long chain carboxylic acid alcohol and mixtures thereof.
A permutation of these aqueous micellar delivery systems are microemulsions which form easily, even spontaneously, in the presence of typically high emulsifier concentrations. Microemulsions are particularly useful as delivery vehicles because a range of materials can be contained therein that would otherwise be sensitive to water, such as hydrolysis sensitive materials. In typical pharmaceutical microemulsion applications, the hydrophobic material is a water insoluble organic material that is emulsified by surfactants to form a discontinuous phase in a continuous aqueous phase (see, for example, U.S. Pat. No. 5,952,004 to Rudnic, et. al.). Such microemulsions can be extremely stable and can provide a useful delivery means. For example, pharmaceutical agents may be dispersed into the hydrophobic material and delivered as part of the aqueous emulsion.
Emulsion technology is also used to create polymeric dispersions wherein a monomer is first emulsified in an aqueous surfactant solution and then polymerized. The resulting emulsion polymers, commonly referred to as latexes, have found many uses including paints and coatings. In order for a latex to spread across the substrate surface and form a uniform coating, it is necessary for it to “wet” the substrate to which it is applied. “Wetting” results when the contact angle, θ, between the aqueous latex and the solid substrate is less than about 90 degrees. Spontaneous wetting occurs when the surface energy between the solid and liquid, γSL is less than the surface energy between the solid and air, γSA. The relationship between these parameters and the liquid-air surface energy, γLA, is given by the relationship below:γSL=γSA−γLA*cos(θ)This relationship is very important when trying to coat a low surface energy substrate (low γSA), such as for example, materials with a surface energy below about 40 dynes/cm, because a very low γSL is required.
One low surface energy substrate of particular interest is polytetrafluoroethylene (“PTFE”) and microporous polytetrafluoroethylene. Due to the inherent hydrophobicity of PTFE, membranes of these materials are of particular interest when in the form of repellent products such as rainwear. Expanded microporous, liquid waterproof polytetrafluoroethylene materials, such as those available from W. L. Gore and Associates, Inc., sold under the trademark GORE-TEX®, as well as expanded PTFE products available from other suppliers are especially well suited for this purpose. The expanded PTFE materials are liquid waterproof, but allow water vapor, in the form of perspiration, to pass through. Polyurethanes and other polymers have been used for this purpose also. To confer good flexibility and light weight in the materials for use in the textile sector, the microporous layer should be made as thin as possible. However, a thinner membrane will generally mean a loss of performance, and thin coatings run the risk of decreasing water repellency.
Low surface energy substrates have historically been coated by solutions having a low γLA and low contact angle. Suitable coating processes for microporous low surface energy materials are described in the art, many of which rely on solvents to wet the desired substrate. For example, EP 0581168 (Mitsubishi) describes the use of perfluoroalkyl methacrylates and perfluoroalkylethyl acrylates for porous polyethylene and polypropylene membranes. These substances are held in physical contact with the surface of the polyolefin porous membrane. The fluorinated monomer or fluorinated monomer and a crosslinking monomer together with a polymerization initiator are dissolved in a suitable solvent to prepare a solution. For example this solution typically can comprises 15% wt. monomer and 85% wt. acetone. After coating, the solvent is vaporized off. The situation is similar with a process for treating the surfaces of polymers with essentially pure solvent solutions containing low concentrations (e.g. less than 1.0% wt.) of amorphous fluoropolymers (WO 92/10532). Likewise, solutions of fluorine-containing polymers are also involved in a patent for coating ePTFE with an amorphous copolymer of tetrafluoroethylene (EP 0561875). In each of these cases, significant quantities of solvent are released during the coating coalescence process. These solvent emissions are both costly and environmentally undesirable. Moreover, solvent-based wetting systems have the inherent limitation of incompatability with a broad range of aqueous fluoropolymers, and the concentration of solvent necessary to wet the substrate limits the amount and type of additive that can be coated on that substrate.
Efforts have been made to convert from these solvent based coating systems to aqueous coatings systems. However, the challenge of achieving stability of the wetting package and to achieve fast wetting speed are hard to meet. One relatively common approach is to add a water soluble organic solvent to the aqueous coating solution or latex. U.S. Pat. No. 6,228,477 teaches a means to coat a low surface energy, microporous PTFE substrate with an otherwise non-wetting, aqueous fluoropolymer dispersion through the use of significant percentages of isopropanol (“IPA”). In one such example, the non-wetting, aqueous fluoropolymer dispersion was diluted to 25% dispersion and 75% IPA, applied to a microporous PTFE substrate, and the solvent evaporated off to thereby form a uniform coating of the desired fluoropolymer. This process unfortunately requires the use of large amounts of IPA and creates significant environmental problems. In other examples in this patent, a number of fluoropolymer treatments were shown to be unstable with high concentrations of water soluble alcohol, further limiting this IPA wetting system.
Aqueous microemulsion systems have been developed to circumvent the need for high levels of VOC's in order to wet low surface energy substrates. One such system that does not require the use of IPA or any other VOC's is taught in U.S. Pat. No. 5,460,872, to Wu et. al. This patent teaches the use of fluorinated surfactants to lower the surface energy and contact angle with microporous PTFE as a means to produce a uniformly coated microporous PTFE substrate. After application of this aqueous dispersion, the fluorinated surfactant and the residual water were then removed by heating.
High costs of manufacturing and potential environmental issues with these prior art materials have highlighted the continuing need for a solution to effectively coat low surface energy substrates without high levels of VOC's or undesirable fluorosurfactants.